This invention relates to turbines and, more particularly, to adjustable turbine vanes for use therein.
A dynamic machine which introduces mechanical work into a gas flow is generally called a "fan" or a "compressor", the latter term most appropriately applied to those machines which add a relatively large amount of energy per unit weight. Conversely, a dynamic machine which extracts mechanical energy from a mass or fluid flow is called a "turbine". A sophisticated dynamic machine, such as a gas turbine engine, generally includes a compressor and a turbine, the gas (usually air) being first introduced into the compressor where work is done in the form of compression. This compressed air is subsequently mixed with a highly combustible fuel and burned in a combustor section, thus considerably increasing the energy of the resultant mixture of combustion products (gases). This gas then passes to the turbine section where mechanical energy is extracted. In such a turbine, the gases pass through a stationary portion called a "vane" or, in some cases, a "stator". Where, as in a gas turbine engine, the gas paths comprise an annulus of substantially constant distance from the engine center line, the machine is called an "axial flow turbine" and the stationary part comprises a plurality of vanes, arranged as spokes of a wheel, the spokes spanning the annulus and constituting a vane stage. After the gas passes through the vane stage, it flows through a rotating stage of blades called the "rotor", or a stage of "buckets" in the particular case of a turbine. A turbine stage comprises one stationary stage of vanes and one rotating stage of buckets.
In a gas turbine engine, therefore, energy is added to the gas by the compressor and by the combustion process, while energy is extracted by turbine. The turbine bucket stage is drivingly connected, by shaft connection, to the associated rotating rotor stages of the compressor. Since the energy available to the turbine far exceeds that required to maintain the compression process, the excess is exhausted through a nozzle at the rear of the engine to produce thrust.
Advanced gas turbine engines incorporate a fan to improve subsonic performance, the fan constituting, in essence, a shrouded propeller. The fan is rotatably driven by a second turbine (or a second plurality of turbine stages) through shaft connection, the shaft being generally concentric with the aforementiond shaft connecting the first turbine and the compressor. (This second turbine is denominated the "low pressure" turbine since it drives the low pressure [compression] fan. The turbine which drives the high pressure compressor called the "high pressure" turbine.) The fan serves to pass a large volume of air around the engine thereby increasing overall engine thrust. In fact, in recent commercial fan engines, the fan moves several times as much air as is taken in by the engine compressor.
Since the fan and compressor are on separate concentric shafts and are driven by separate, axially displaced turbines, a means of regulating their relative rotating speeds is required for engine performance optimization. Further, it becomes necessary to control the relative amounts of energy added by the fan and compressor, which in turn is controlled by how much energy is extracted by their respective turbines. It can be appreciated that the faster the machinery rotates, the more air it pumps, and vice versa. It can also be appreciated that a stage of turbine vanes may function as a set of louvers, or "venetian blinds", if constructed so as to be adjustable about their longitudinal axes. Thus, by opening and closing the vane stage, the amount of flow passing therethrough can be varied, thereby modulating the energy extraction characteristics of the turbine (either the high or low pressure turbine). Further, if a stage of vanes is adjusted in the high pressure turbine, for example, the direction of the gas flow exiting that stage will also be varied. This effects the energy extraction capability of the following stage of rotating buckets, much in the way the lift of the wing changes as its orientation angle (angle of attack) is varied. If the capability of the subject high pressure turbine buckets to extract energy was thus reduced, more energy would be available to the low pressure turbine and the fan would speed up relative to the compressor. This ability to regulate the relationship between the fan speed and compressor speed is extremely important in designing the most efficient engine for many operating conditions (such as takeoff, climb, cruise, etc.).
One problem facing the designer of an adjustable vane is to develope a mounting arrangement which will position the vane radially with respect to the turbine casing and prohibit radial movement therebetween which would cause the vane to rub and bind on the turbine casing wall. It is advantageous to cantilever the vanes off the outer turbine wall since this eliminates half of the vane mounting structure, which could disrupt the fluid flow path and thereby cause aerodynamic losses and inefficiency. However, this compounds the problem of the vane mount on the outer wall since it alone must now prevent both inward and outward radial movement.
Further, as the turbine vanes are caused to move (adjusted so that the flow area is modulated) there must be sufficient clearance between the vanes and the turbine casing walls to allow for the differing thermal expansion rate of the vanes and casing, so that the vanes may be freely moved (by some actuation means) during operation of the gas turbine engine. A secondary flow of gases is thereby established from the high pressure side of the vanes to the low pressure side thereof, the path being across the end or tip of the blade through the aforementioned clearance. This secondary flow causes undesirable efficiency losses. The designer is further faced with the problem of sealing this clearance without causing vane binding during actuation, while providing a seal which will withstand the hostile, high temperature environment of a gas turbine engine turbine.
Prior state-of-the-art attempts to solve these problems have been directed toward the incorporation of simply supported, adjustable turbine vanes (supported at both ends rather than cantilevered) which invariably results in an inefficient "stepped" flow path. Typical of the simply supported vane assemblies is the patent to Thenault et al, U.S. Pat. No. 3,314,654, which is assigned to the same assignee as the present invention. Attempts at providing adequate tip sealing include flexible members affixed to the tips as typified by the U.S. patent to Willi, U.S. Pat. No. 2,776,107; "Inflatable Rubber Tubes Affixed to the Tips" as represented by Japanese Utility Model Application Publication No. 10,710/42; and, by "Internal Pressure Actuated Floating Seals" as shown in the patent to Mayo, U.S. Pat. No. 3,601,497. However, when a cantilevered vane is adopted for use in a high temperature turbine wherein the free end of the vane is proximate a hub rotating at extremely high speed, such seals cannot withstand the hot gas environment or the friction created by the rotating hub.